The present invention relates generally to well tools which are used to block the flow of fluids or gasses through the annular space between tubing inserted into the well bore and the wall of the well bore by sealing off the space between them. More specifically the present invention relates to packers which are particularly useful in various wells having normal-to-high temperature well conditions as well as in hot steam injection wells, geothermal wells, and deep wells with high pressure. In addition, the packers of the present invention may also be utilized in wells with a corrosive environment.
For greater packer reliability it is essential that well bore packers have a sealing element which will establish and maintain a seal under producing conditions. Most conventional packers are reliable in wells in normal-to-medium temperature ranges and with relatively low pressure. The packer sealing elements are normally formed of an elastomeric material such as rubber. Typically, when a packer is set, the elastomer sealing element is compressed longitudinally and expanded radially to form a seal against the well casing. Methods for setting this sealing element as well as the slips in packers include the application of an upward or downward force on a tubing string, actuation by hydraulic pressure, and/or rotation of the tubing string.
However, for wells having high temperatures and high pressures, the reliability of these conventional packers is poor. Under these conditions, most elastomer seals become brittle, deteriorate or swell and lose sealing capability. Such failure is hastened by the thermal cycling that occurs, for instance, due to periodic interruptions in the steam supply. This would result in expansion and contraction of the casings and tubings, which expansions and contractions is the lower end of the tubing relative to the casing. Such shifting can vary from several inches to several feet, depending upon the particular well structure. This relative shifting between the lower ends of the casings and tubings is a factor which most conventional packer constructions are not designed for and which they cannot withstand.
Other problems associated with many conventional packers is that of leakage due to high pressures within the well bore being sealed. For example, in a typical hydraulic set packer, if the pressure within the well exceeds the effective hydraulic setting pressure, leakage past the packer seal may occur. Present packers experience great difficulty in providing an effective seal against such leakage without the application of excessive internal setting pressure. Such packers are also complex and costly to operate. Therefore, sealing elements of conventional packer assemblies are found to be not satisfactory for application in the wells having high temperatures as well as with high pressures.
Many solutions have been offered for packers at high temperatures and/or at high pressures. These have involved the application of packing elements which are constructed of fluoroelastomer (plastic) and asbestos material and are relatively difficult to mechanically compress. Various packers using such a sealing element are known. For example, see U.S. Pat. Nos. 3,381,969; 4,176,715; 4,258,926 and 4,281,840. However, such solutions are not totally corrective of the problem. At high temperatures, these sealing elements will not hold high differential pressure due to unacceptable mechanical properties of the thermoplastic when subjected to compression load to initiate the desired seal. Therefore they are inherently deficient in maintaining a leakproof joint over extended periods of time. Another disadvantage is that under high temperature conditions it is often difficult to prevent elastomeric and synthetic resin materials from being extruded from even very small clearance in the annular space.
Packing made of woven asbestos and Inconel wire has also been used for packing elements at temperatures above 500.degree. F., see U.S. Pat. No. 4,281,840. However, when subjected to high differential pressures, leakage occurs to an extent tolerable in steam injection wells but excessive for many geothermal applications.
Other packers in the prior art have a metallic seal assembly which can be deformed and expanded radially to form a sealing engagement with the interior of the casing, see U.S. Pat. Nos. 3,389,918 and 3,472,520. However, in some cases the method of deformation of the sealing element and its construction may result in such packers providing only a very limited surface area of engagement so that the packers may be vulnerable of leakage due to high pressures within the well bore being sealed.
Yet another form or type of packer construction involves an annular metal seal ring of soft and malleable lead, see Great Britain Pat. No. 2,074,630A. This type of packer is limited in its application to low temperatures, since the melting point of lead is very low.
U.S. Pat. No. 4,127,168 discloses a packer having a plurality of frustoconical resilient metal seal rings. U.S. Pat. No. 4,178,992 discloses a metal seal plug for sealing or packing off the bore of a tubing string. Both patent describe the advantages of metal to metal seals over conventional elastomeric seals under certain conditions, such as high H.sub.2.spsb.S concentration or high temperature well fluids. These packers are set in the well bore hydraulically, a means for compressing and radially expanding the metal seal ring to contact the interior of the casing string. However, problems of transmitting the necessary force to the packer are typically encountered with the hydraulic methods used by most of these packers. Hydraulic actuation has the problem of pressure fluctuation with consequent repeated releasing and resetting of the slips, often resulting in casing damage, movement of the packer and loss of sealing ability. Moreover, for setting the packer by this hydraulic method which is often with application of unnecessary excessive internal setting pressure is not a simple way and the work necessary for this purpose is very time consuming and grueling. Sometimes, to work and setting the packer can only be accomplished by the considerable effort of several people.